Color image reproduction systems known in the art permit images which have been recorded on photographic film to be scanned at a multiplicity of points, and converted to digital red (R), green (G), and blue (B) values. These digital values may be manipulated by a digital computing means to achieve improvements in the image characteristics which they represent, and then they may be converted to a visible form such as a print, transparency, or an image on a video monitor using any one of a number of digital image reproduction means known to the art. For example, U.S. Pat. No. 4,500,919 entitled "COLOR REPRODUCTION SYSTEM" by W. F. Schreiber discloses an image reproduction system of one type in which an electronic reader scans an original film based color image, which may be in the form of a transparency or print, and converts it to a digital image. A computer workstation and an interactive operator interface, including a video monitor, permit an operator to edit the image by means of displaying it on the monitor. When the operator has composed a desired image on the monitor, the workstation causes the output writer device to make an inked output of the reproduced image.
The present invention may be used in such a system, but is not limited to a system with an interactive operator interface. It may also be used in an automatic system in which the original film based color image is scanned and the analog signals so generated converted to a representative digital value, processed by a pre-selected set of algorithms, and output to a writer which makes a reproduced image or output to an image storage device for later reproduction using by any of a number of devices.
Using color negative or transparency film as the original capture media carries with it some benefits not normally available to other capture media such as video and "instant" photographic products. Among the benefits of using a color negative film is its long exposure latitude, which may exceed an exposure range corresponding to 1.8 relative log exposure, or six camera stops. Cameras which are designed primarily to record images using negative film often take advantage of this latitude to simplify their exposure control requirements. Among the advantages of a color transparency film is its large dynamic range which may exceed a transmittance ratio of 1000 to 1. Images captured on transparencies retain a natural look not achieved by other lower dynamic range media.
It may be noted that most natural scene elements modify the incident illumination by reflecting a fraction of the light. In ordinary circumstances, this fraction is primarily a property of the object and very little a property of the level of the illumination. Because of this, the human visual system has developed the ability to recognize the reflectance of objects even when illuminated by very different types and levels of illumination. Image recording and reproduction systems are required to record scenes under similar variations in the type and level of the illumination. In order to preserve a tone scale which is satisfying to a human observer, they must preserve at least the relative reflectances in the final reproduction of the image. In so doing, they may be required to adjust the range of the input image to accommodate the output imaging medium's dynamic range, and they may be required to modify the contrast of the input image to accommodate the output imaging media and viewing conditions.
Adjusting the input range to accommodate the output range while preserving reflectances, requires multiplication in the scene relative exposure space, while adjusting the contrast would require exponentiation. However if the logarithm of the relative scene exposure is used, then adjusting the range (i.e., rebalancing) can be done by additions, and contrast changes may be done by multiplications.
As is well known to those skilled in the art, the relationship between the exposure received by an individual area of photographic film and the subsequent transmission of that area after development is usually expressed graphically using logarithmic axes. Considering the foregoing, this expression of the transmission of the film in terms of its density offers a number of obvious advantages, especially when the image on film is converted to digital form by scanning and processed by a computer means.
First, over the majority of its range, the relationship between log relative scene exposure and density is approximately linear. This means that the digital images stored as densities may be rebalanced by simple additions (or subtractions), and they may have their contrast adjusted by multiplications while preserving the reproductions of at least the relative scene reflectances. This avoids a distortion of the output image's tone scale.
Second, since the human visual system's contrast sensitivity is more nearly logarithmic than linear, the use of density is a more efficient means of digitally coding the image data. This means that fewer digital bits are required to preserve the high quality image that is recorded in the negative or positive film.
However as is also known to those skilled in the art, the relationship between the logarithm of the exposure and the density formed in the film is not even reasonably linear near the ends of the scale; that is in the "toe" and "shoulder" region of the curves. This fact limits the range over which the tone scale of an output image may be reproduced without distortion both in contrast and in color balance.
The shape of the toe and shoulder may be different for different emulsion formulations. So, for instance, the shapes of the toes and shoulders for negative films are distinctively different from the shapes for transparency films. Furthermore, the shapes for different types of negative films may be different for different emulsions, for instance, KODAK EKTAR25 has a different toe shape than does KODAK GOLD400 film. The same is true for different types of transparency films.
Beyond the differences in shape, the absolute level of the minimum density found within different samples of the same emulsion can change depending on the conditions under which the film was used and processed. This difference can be approximately described as a shift of the emulsion's density vs log exposure curve up and down the density axis.